LECTURE XXI. 



THE homology of parts and the correspondence of stages in the 

 life-histories of two divergent organisms are often brought to light 

 or made more apparent by comparison with a third organism which 

 is intermediate between the two. These intermediate organisms 

 are often spoken of as links or synthetic forms. 



It is owing to its character as such a connecting-link between 

 the Ferns on the one hand and the Seed Plants on the other that 

 the study of the structure and life-history of Selaginella is of par- 

 ticular interest. 



There are large numbers of different kinds of Selaginella. Only 



FIG. 44. Selaginella martensii. a, lateral leaves ; b, dorsal leaves ; c, cones. 



one of them is native to the British Isles. As it is rare and diffi- 

 cult to obtain, our study will be carried out on a tropical species, 

 Selaginella martensii, which is usually grown in green -houses 

 in this country. 



This Selaginella is a low-growing, delicate and moss-like plant. 

 Its white translucent stems lie prostrate on the soil, branching only 

 in one plane that parallel to the surface of the soil. The ends of 

 the branches are closely covered with green leaves. The leaves 

 are oval in outline, but pointed. Their edges are serrated. A mid- 

 rib runs from the tip to the base of each. They are attached directly 

 to the stem, having no stalk. They are distinctly of two sizes and 



170 



